1. Field of the Invention
The present invention is directed to an x-ray image intensifier for an x-ray diagnostics installation.
2. Description of the Prior Art
Known x-ray image intensifiers generally include a vacuum vessel having an input luminescent screen and a photocathode at one end face thereof, and electron optics, fed by a voltage source, for generating an electrical field which focuses the electrons emitted by the incidence of x-radiation at a point of the input luminescent screen/photocathode onto a corresponding point of an output luminescent screen, disposed at an opposite end face of the x-ray image intensifier.
X-ray image intensifiers are used in x-ray diagnostics to convert an x-ray shadow image, produced by transillumination of a patient with x-rays, into a visible image, and to intensify the image. A video camera tube, whose output signals are supplied to a monitor via a video chain, is connected to the output of the x-ray image intensifier. The examination region is displayed as an image on the monitor.
A known x-ray image intensifier of the above type is described in the text "Das Roentgenfernsehen", Gebauer et al., 1974, pages 54-56. The electrode system in this known device has a plurality of cylindrical or annular electrodes having respectively different diameters. A different voltage is applied to each electrode for generating an electrical field for focusing the electrons produced at a point of the input luminescent screen/photocathode onto a corresponding point of the output luminescent screen. Due to the high voltage differences of the consecutive electrodes required for the deflection of the electrons, large gradients in the electrical field are caused, particularly in the proximity of the photocathode. This results in disturbances in the electron trajectories. In particular, these disturbances causes distortion errors in the edge region of the output luminescent screen, and degrade the modulation transfer function of the system. These errors can only be compensated with significant complexity, by suitable design of the shape of the electrodes, and by increasing their number.
An x-ray image intensifier is described in U.S. Pat. No. 3,688,146, wherein the focusing electrodes are applied to the inside of the tube envelope as a metal coating, on regions of the tube envelope having differing diameters. Again, the high differences in potential in the region of the electrodes disturb the electron trajectories.
Another type of x-ray image intensifier is disclosed in British application No. 839 681, wherein a focusing electrode is applied to the inside of the tube envelope as a metal coating; however, for reducing large potential gradients, a semiconductive layer is applied to the inside of the tube envelope between a focusing electrode and the anode. In a further embodiment also disclosed in this published application, a layer of semiconductive material may also be applied to the focusing electrode of the image intensifier, to serve as a getter for free cesium iodide. The semiconductive material is provided in additon to the focusing electrode.